Our present invention relates to a method of drying a filter cake and to a press for carrying out that method. More particularly, the invention relates to a filtration method which involves the drying of filter cakes formed in filter chambers of a filter press and to a filter press for carrying out that method.
Filter presses using membrane filter elements and in which a filter cake is formed in each of the filter chambers and the membrane can be pressed by fluid pressure against the filter cake to press liquid therefrom are used for the separation of liquids from solid matter in suspension. The filter cake liquid which is expressed from the filter cake may be the original suspending liquid and/or liquid which is contacted with the solids in a washing operation, the washing liquid being pressed out of the filter cake at least in part by the membrane.
The filter cake can then be heated and the filter chamber evacuated.
EP 0 676 225 describes a filter plate for a multichamber filter press which can carry out such a process. However, the energy consumption and time required to reach a certain residual moisture content of the filter cake is relatively high. As a consequence a separate drying apparatus usually must be provided in order to achieve the desired reduction in the moisture content of the solids or the increase of the solids content of the product.
It is the principal object of the present invention, therefore, to provide an improved method of operating a filter press which can lead to a higher degree of drying of the filter cake than has been possible heretofore without significant additional cost with respect to energy and capital cost.
Another object of this invention is to provide a method of drying the filter cake in a multichamber membrane-tight filter press which can lead to a dryer filter cake, i.e. a filter cake with greatly reduced liquid content and increased solids content without a need for expensive materials with high temperature resistance.
Still another object of the invention is to provide an improved multichamber filter press utilizing membrane filter elements which can be operated more efficiently from an energy-consumption point of view and which does not need materials resistant to high temperature to achieve a high degree of dryness.
Yet another object of this invention is to provide a filter press which is free from drawbacks of earlier multichamber filter presses.
These objects and others which will become apparent hereinafter are achieved, in accordance with the invention by heating the filter cake exclusively from the side thereof turned away from the membrane filter element and under such conditions that the temperature, considering the reduction of the boiling point of the liquid phase resulting from the application of vacuum, is sufficiently high as to generate a vapor layer which advances through the filter cake with a vapor layer front that is substantially planar and uniform so that the vapor layer penetrates through the filter cake and drives out the residual liquid of the filter cake toward drainage surfaces of the unheated membrane. The heating is carried out over the area of the filter cake so uniformly that the transition zone between the vapor phase and the liquid phase is planar, homogeneous and advances through the filter cake without liquid breakthrough of the vapor.
More particularly, the method of the invention for separating solids from a liquid of a suspension in filter chambers provided with respective membrane filter elements and in which a respective filter cake is formed in each chamber and is subjected to an aftercompression phase by the respective membrane filter element, in its drying operation, can comprise the steps of:
(a) heating each filter cake along a supported side thereof opposite a side along which a respective membrane filter element is provided;
(b) simultaneously subjecting the filter cakes to a vacuum to reduce a boiling point of the liquid in the filter cakes; and
(c) controlling the temperature to which each filter cake is heated at the supported side thereof so that a vapor layer is formed at the supported side and is propagated uniformly through the filter cake to drive residual liquid out of the filter cakes toward drainage surfaces of respective unheated membrane filter elements, the heating being effected over the entire area of each filter cake so that a transition zone between each vapor layer and the respective residual liquid of each filter cake transits through the filter cake homogeneously in a plane and without local vapor breakthroughs.
The advantage achieved with the method of the invention is a quantitatively higher degree of removal of the residual liquid because the latter is driven out mechanically by the advance in the vapor phase. The advancing vapor phase thus acts synergistically with the action of the vacuum and vice versa to permit the removal of the residual moisture, the vacuum simultaneously reducing the boiling temperature of the liquid phase, thereby eliminating the need for expensive high-temperature materials in the region of the membrane filter element. While the residual moisture is usually water, the invention is applicable to liquids which may be organic or inorganic solvents, oil and other liquids.
Preferably the vacuum is applied commencing with the afterpressing phase of the press operation, i.e. before commencement of heating, thereby further accelerating the drying process. It has also been found to be advantageous to apply the vacuum intermittently rather than continuously and with intervals of durations between intervals of vacuuming applications which can depend upon the solids which are recovered and the liquids which are removed. It has been found to be advantageous, moreover, to begin the heating of the filter cake at its side remote from the membrane filter element during the afterpressing phase as well.
In order to achieve optimal heating and a high heat transfer, the filter cake can lie directly against the surface of the heating element.
It has been found to be advantageous with respect to the high heat-transfer rate, to maintain the filter cake during the drying under the afterpressure applied by the membrane, the afterpressure being adjusted to be greater than the vapor pressure of the filter cake liquid. This stabilizes the filter cake in the filter chamber and compensates for the shrinkage of the filter cake as the residual liquid is removed so that the deterioration of the filter cake and formation of cracks or crevices therein through which vapor breakthroughs can develop, are avoided.
It has been found to be advantageous further to provide the membrane pressurization medium so that it is a heating or cooling fluid and thus can adjust the temperature differential across the filter cake. This allows optimization of the drying process since it enables the temperature difference across the filter cake to be adjusted or the temperature at the drainage side of the filter cake to be matched to the particular requirements of the process. As noted, the fluid may act as a heating fluid or as a cooling fluid.
From the apparatus point of view, the filter press can comprise a plurality of filter chambers separated by respective heating plates and each provided with a respective membrane filter element which can be covered by a respective filter cloth. The membrane filter element, in turn, can flank respective support plates which likewise separate the chambers from one another, the support plates forming frames at which the membranes are affixed thereto. A suspension inlet may extend through the support plates and the heating plates to feed the suspension to the respective chambers and drainage passages may be formed at the sides of the membranes.
The membranes can be sealed at their edges to the support plates or can be integral therewith and seals at the periphery may be provided between each frame and the respective heating plate. The filter cloths can be clamped to the frames along the peripheries thereof and the membranes can have drainage channels for the filtrate beneath the respective filter cloth. In any case between each two membrane filter elements, a respective heating element or plate is provided and over the entire periphery of the filter surface uniformly distributed filtrate run-off bores can be provided.
To ensure a sufficiently large flow cross section for the discharge liquid, especially because of the use of a vacuum to promote the discharge of the liquid, it has been found to be advantageous to provide in corner regions of the membrane filter elements, filtrate runoff passages which have a configuration optimized as to cross sectional area, preferably a triangular cross section.
With a view to high operating efficiency it is advantageous to provide the plate frame with a vacuum-tight edge seal and to integrate the filter cloth clamp within the edge seal.
To obtain heat transfer which is as good as possible from the heater to the filter cake, the heating elements can be plates provided from a material with high thermal conductivity and/or high chemical resistance.
The shape of the heating elements can vary widely. However, the heating element on its side turned toward the filter cake and hence toward the membrane filter element of the respective chamber can be planar or can be dished to form a cake-receiving depression and the surface of this heating plate can be free from drainage channels. It is however also possible to provide the surface in contact with the filter cake of the heating plate with a contour or profiling so that the contact area is increased for greater heat transfer without undesirably increasing the adhesion force of the filter cake to the surface of the heating element. In general that adhesion force is substantial in any event.
To facilitate the separation of the filter cake from the heating element or plate following the drying operation, it is advantageous to provide the surface element of the heating element at least in the region in which it contacts the filter cake with an antiadhesive coating, preferably of polytetrafluoro-ethylene (PTFE). The surface can have high or bright polish for this purpose as well.
The heating elements themselves can be formed in a single piece or can be composed of one or more pieces and the parts of the heating element can be cemented, welded or bolted together, e.g. with the aid of seals. It is also possible to provide the heating element with round and/or oval and/or rectangular heating passages and other cross sectional shapes are conceivable as well. The heating elements can be formed by machining or by casting and it is possible to provide the heating elements with electrical heating conductors although in this case corresponding heating sheets which can be composed of metal fabric can be used as well. What is important of course is that the heat be emitted from the heating plates or elements uniformly or homogeneously and this can be ensured by making the heating passages in the configuration of a meander.
Finally the heating elements can be thermally insulating in the edge region of the frames and the thermal insulation material can avoid radiating heat to all surfaces which are not in contact with the filter cake.